Image forming apparatus

Abstract

When a time from when a point of the surface of the charging member contacts the cleaning member until the point subsequently contacts the cleaning member again by a rotation of the charging member is defined as a rotation time A [sec], and a time constant τ of the charging member is a value represented by τ=½πfp [sec], wherein fp is a frequency at which an absolute value of an imaginary part obtained by impedance measurement of the charging member shows a maximum value, and the time constant τ is defined as an index indicating that an electric charge of the surface of the charging member is attenuated, the rotation time A and the time constant τ satisfy the following relationship: A/τ≥60, wherein the time constant τ is 1.0×10 −5 [sec] or more and 1.2×10 −3 [sec] or less.

What is claimed is: 
     
         1 . An image forming apparatus comprising:
 a rotatable photosensitive member;   a charging member configured to electrically charge a surface of the photosensitive member while rotating in a predetermined direction in contact with the surface of the photosensitive member; and   a cleaning member configured to clean a surface of the charging member in contact with the charging member,   wherein when the surface of the photosensitive member is charged by the charging member, in a case where   a time from when a point of the surface of the charging member contacts the cleaning member until the point subsequently contacts the cleaning member again by a rotation of the charging member is defined as a rotation time A [sec], and   a time constant τ of the charging member is a value represented by τ=½ πfp [sec], wherein fp is a frequency at which an absolute value of an imaginary part obtained by impedance measurement of the charging member shows a maximum value, and the time constant τ is defined as an index indicating that an electric charge of the surface of the charging member is attenuated,   the rotation time A and the time constant τ satisfy the following relationship:
     A/τ≥ 60, 
   
       wherein the time constant τ is 1.0×10 −5  [sec] or more and 1.2×10 −3  [sec] or less. 
     
     
         2 . An image forming apparatus according to  claim 1 , wherein the time constant τ is 1.0×10 −5  [sec] or more and 1.0×10 −4  [sec] or less. 
     
     
         3 . An image forming apparatus according to  claim 1 , wherein the charging member is constituted so as to be rotated with rotation of the photosensitive member. 
     
     
         4 . An image forming apparatus according to  claim 1 , wherein the cleaning member is rotatable. 
     
     
         5 . An image forming apparatus according to  claim 4 , wherein the cleaning member is a roller including a rotation shaft and an elastic layer formed around the rotation shaft in a cylindrical shape. 
     
     
         6 . An image forming apparatus according to  claim 4 , wherein the cleaning member includes a cleaning portion for cleaning the surface of the charging member in contact with the charging member and a non-cleaning portion smaller in diameter than the cleaning portion and being in non-contact with the surface of the charging member, and
 wherein the cleaning member is constituted so that the cleaning portion and the non-cleaning portion alternately oppose the point of the surface of the charging member with respect to a rotational axis direction of the charging member.   
     
     
         7 . An image forming apparatus according to  claim 4 , wherein the cleaning member is a roller including a rotation shaft and an elastic layer formed around the rotation shaft in a helical shape. 
     
     
         8 . An image forming apparatus according to  claim 4 , wherein the cleaning member is constituted so as to be rotated with rotation of the charging member. 
     
     
         9 . An image forming apparatus according to  claim 1 , wherein in a cross section substantially perpendicular to a rotational axis of the charging member, when
 a rectilinear line passing through a rotation center of the photosensitive member and a rotation center of the charging member is a rectilinear line L 1 , and   a rectilinear line passing through a rotation center of the cleaning member and the rotation center of the charging member is a rectilinear line L 2 ,   the cleaning member is provided so that the rectilinear line L 2  is positioned on a side upstream of the rectilinear line L 1  and downstream of a contact position between the charging member and the photosensitive member with respect to a rotational direction of the charging member on the basis of the rotation center of the charging member.   
     
     
         10 . An image forming apparatus according to  claim 1 , further comprising:
 a first bearing member configured to rotatably support the cleaning member and being movable;   a second bearing member configured to rotatably support the cleaning member and movably supported by the first bearing member;   a first urging member configured to urge the first bearing member toward the photosensitive member; and   a second charging member configured to urge the second bearing member toward the charging member and supported by the first bearing member.   
     
     
         11 . An image forming apparatus according to  claim 1 , further comprising a cartridge including the photosensitive member, the charging member, and the cleaning member,
 wherein the cartridge is configured to be demountable from the image forming apparatus.